feat: apt's proxy through AptProxyMixin (NetOrk/netork#1397)

get_package_proxy() is what apt-config dump says, not 00proxy alone: a
proxy in 90curtin-aptproxy or /etc/apt/apt.conf wins over it and was
missed. The driver now supplies only the transports: reads on the exec
channel, root commands through _root_outcome, the file through the staged
_install_file_as_root. New: get_package_proxy_report() and
check_package_proxy(). configure_package_proxy() also comments out other
proxy lines, reads the result back and fetches through the proxy.

Requires napalm-device-types 4.22.0; version 0.14.0.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- apt's proxy through napalm-device-types' `AptProxyMixin` (NetOrk/netork#1397). `get_package_proxy()` is what `apt-config dump` says, not `00proxy` alone: a proxy in `90curtin-aptproxy` or `/etc/apt/apt.conf` wins over it, and was missed. New: `get_package_proxy_report()` (every line that sets it, per-host overrides, the https sources) and `check_package_proxy(url, expect)` (a fetch with `apt-helper`, past any proxy). `configure_package_proxy(url)` now also comments out other proxy lines, reads the result back and fetches one `InRelease` through the proxy; it fails when apt still fetches elsewhere. Reads run on the exec channel. Requires napalm-device-types 4.22.0; version 0.14.0.
 - `get_host_firewall()` (napalm-device-types' `HostFirewallMixin`, NetOrk/netork#1344): the host's own netfilter rules — `nft -j list ruleset`, with the tables iptables manages taken from `iptables-save` — as chains and rules in one shape. Read-only, as root, on an exec channel (the nft JSON is one long line). Requires napalm-device-types 4.21.0; version 0.13.0.
 - `get_service_packages()` (napalm-device-types' `ServicePackagesMixin`, NetOrk/netork#1236): which installed packages each systemd service is made of — the one shipping its unit file and the one shipping its `ExecStart` binary. Read-only, on an exec channel under `sh`, without sudo. Requires napalm-device-types 4.20.0; version 0.12.0.